ELEMENTAL AND MINERAL STRATIGRAPHY OF MIXED CARBONATE-CLASTIC SYSTEM: MIDDLE JURASSIC GYPSUM SPRING FORMATION, BIGHORN BASIN, WYOMING

X-Ray diffraction (XRD) and x-ray fluorescence (XRF) analyses aid in refining stratigraphic correlations across the mixed carbonate-clastic Middle Jurassic Gypsum Spring Formation. Variability in elemental and mineral content of the units supplement traditional lithostratigraphic interpretations of paleoenvironmental change through time and space. The Gypsum Spring has been a challenge to correlate due to lithologic similarity of units and thick, monotonous mudstone sequences barren of diagnostic fossils.

The Gypsum Spring Formation was deposited along the margins of a cratonic forebulge in a retro-arc foreland basin associated with the Nevadan Orogeny. The Gypsum Spring Formation is divided into a lower unit of gypsum, shales and siltstones, a middle unit of carbonates and variegated shales, and an upper unit primarily consisting of shale and siltstones.

XRF analyses of 284 samples from nine outcrops define chemostratigraphic zones in the Middle Jurassic Gypsum Spring Formation. Two depositional sequences and ten chemostratigraphic zones are recognized from observed elemental patterns of Al, Si, K, Ca, Ti, Fe, V, Cr and Zr. Preliminary results from XRD analyses of these samples are building a picture of the “mineral stratigraphy” of the Gypsum Spring Formation. XRD is particularly useful in identifying the fine-grained minerals that make up much of the Gypsum Spring Formation. Knowledge of both elemental and mineralogic makeup of the units will also help distinguish between depositional and diagenetic patterns across the basin.